The .alpha.,.beta.-unsaturated aldehyde dihydrocitral (E/Z-3,7-dimethyl-2-octen-1-al) is a valuable intermediate.
.alpha.,.beta.-Unsaturated carbonyl compounds are generally important intermediates for the manufacture of odorants, vitamins and carotenoids (See, for example, Chem. Ztg. 97, 23-28 (1973) and Ch. VI ("Total Syntheses") in "Carotenoids", Ed. Otto Isler, published by Birkhauser Basel and Stuttgart (1971)). Their production by acid-catalyzed rearrangement of .alpha.-alkynols has already been described in the 1920s by K. H. Meyer and K. Schuster, Ber. deutsch. Chem. Ges. 55:819-823 (1922) and H. Rupe and E. Kambli, Helv. Chim. Acta 9:672 (1926). The isomerization of secondary or tertiary .alpha.-alkynols to .alpha.,.beta.-unsaturated carbonyl compounds has also generally become known as the Meyer-Schuster or Rupe-Kambli rearrangement. In the case of the rearrangement of a carbonyl compound having a terminal alkynyl group there are obtained aldehydes, otherwise ketones are the rearrangement products: ##STR1##
wherein R.sup.1 and R.sup.2 each signify hydrogen or an aliphatic or aromatic residue.
Depending on the reaction conditions, the rearrangement of dihydrodehydrolinalyl acetate catalyzed by silver or copper ions yields, according to G. Saucy, et al. (Helv. Chim. Acta, 42:1945-1955 (1959)), a mixture of "allene acetate" (1 -acetoxy-3,7-dimethyl-octa-1,2-diene) and "diacetate" (1,1-diacetoxy-3,7-dimethyl-2-octene), which can then hydrolyze to dihydrocitral: ##STR2##
This rearrangement of dihydrodehydrolinalyl acetate is also known as the Saucy-Marbet rearrangement.
Compared with the synthesis of dihydrocitral starting from isoheptanoyl chloride, described by C. C. Price and J. A. Pappalardo, J.A.C.S. 72:2613-2614 (1950), the Saucy-Marbet rearrangement has the advantages of a higher yield, namely 80% in comparison to 20%, and the avoidance of lachrymatory intermediates. Moreover, the use of a silver- or copper-containing catalyst is disadvantageous.
Further known methods for the production of dihydrocitral are the rearrangement of 3-methyl-1-(3-methylbutoxy)-butta-1,3-diene, which is carried out in biphenyl at 350.degree. C., according to Japanese Patent Publication (Kokai) 203025(1982)/Chem. Abs. 99, 5873r (1983), and the oxidation, which proceeds in 59% yield, of 3,7-dimethyl-oct-2-en-1-ol with silver carbonate on Celite.RTM. [Fetizon's reagent; B. C. L. Weedon and co-workers, J. Chem. Soc. Perkin Trans., 1:1457-1464 (1975)]. The high reaction temperature and, respectively, the use of a silver-containing catalyst are disadvantageous.
An interesting variant of the aforementioned Meyer-Schuster rearrangement has been described briefly by C. Y. Lorber and J. A. Osborn in Tetr. Lett., 37:853-856 (1996); this is the rearrangement of methylbutynol to prenal using, a molybdenum catalyst. In this case, methylbutynol is rearranged to prenal in ortho-dichlorobenzene as the solvent in the presence of the catalyst system molybdenyl acetylacetonate, dibutyl sulphoxide and 4-tert.butylbenzoic acid. Although the yield in this rearrangement is indicated to be 97%, the prenal was not isolated from the reaction mixture, but the stated yield was obtained by gas-chromatographical analysis of the crude product. Presumably, it was difficult to work up the reaction mixture in order to isolate prenal.
L. A. Kheifits and co-workers found that dehydrolinalool could be converted into citral only in 28% yield and into 2-hydroxymethyl-1-methyl-3-isopropenylcyclopent-1-ene in 12% yield at 170.degree. C. in a reaction period of 14 hours when a molybdenum catalyst produced from molybdenum oxide and triphenylsilanol was used for the rearrangement (Tetr. Lett., 34:2981-2984 (1976)).
From the above remarks it is evident that the previously known processes for the catalyzed rearrangement of .alpha.-alkynols to .alpha.,.beta.-unsaturated aldehydes, e.g., dehydrolinalool to citral, have serious disadvantages, which presumably would also apply to the analogous rearrangement of dihydrodehydrolinalool to clihydrocitral.